The traditional reliability of telecommunication systems that users have come to expect and rely upon is based in part on the systems' operation with redundant equipment and power supplies. Telecommunication switching systems, for example, route tens of thousands of calls per second. The failure of such systems, due to either equipment breakdown or loss of power, is unacceptable since it would result in a loss of millions of telephone calls and a corresponding loss of revenue.
Power plants, such as battery plants, address the power loss problem by providing the system with an energy reserve, a backup battery, in the event of the loss of primary power to the system. A battery plant generally operates as follows. The battery plant generally includes a number of backup batteries, rectifiers and other power distribution equipment. The primary power is produced by the rectifiers, which convert an AC mains voltage into a DC voltage to power the load equipment and to charge the backup batteries. The primary power may, however, become unavailable due to an AC power outage or the failure of one or more of the rectifiers. In either case, the backup batteries then provide power to the load. Redundant rectifiers and backup batteries may be added to the battery plant as needed to increase the availability of the battery plant.
A battery plant that powers telecommunications systems such as transmission and switching systems in wireless base stations commonly employs valve-regulated lead-acid (VRLA) batteries as the energy reserve. The backup batteries are typically coupled directly to the output of the rectifiers and may instantly provide power to the load in the event the AC power outage occurs. During normal operation, the backup batteries are usually maintained in a fully charged state to maximize a duration for which the backup batteries can provide energy to the load equipment.
As a backup battery ages, its capacity or energy-storage capability decreases, reducing the duration for which the backup battery can provide energy, even when fully charged. In many telecommunications applications, a backup battery is considered to have failed when its actual capacity has fallen below a threshold, such as 80% of its rated capacity. A failed backup battery should be replaced in an orderly fashion to maintain the availability of the battery plant. It is crucial, therefore, to be able to assess whether the capacity of a particular backup battery has fallen below the threshold.
The capacity of a backup battery may be assessed when the backup battery is on-line or off-line. One straightforward approach is to take the backup battery off-line and couple it to a dissipative-resistive load. The load can then completely discharge the backup battery at a constant current thus providing an accurate indication of the backup battery's capacity. The off-line method, however, requires that the backup battery be temporarily removed from the battery plant, decreasing the availability thereof. Therefore, to maintain the battery plant at the desired availability level, the capacity of the backup battery should be assessed on-line.
The complete discharge method, however, has some major disadvantages. If an AC power outage occurs during or after the discharge test, but before the backup battery has been fully recharged, the full energy reserve provided by the backup battery will not be available, thus jeopardizing the availability of the battery plant and the reliability of the telecommunications system powered thereby. Further, since a backup battery may only be charged and discharged a finite number of times, each cycle of complete discharge and charge necessarily reduces the overall life span of the battery.
Accordingly, what is needed in the art is a system and method for assessing the capacity of a backup battery that provides an accurate measurement of the backup battery's capacity yet maintains the availability of the battery plant at a satisfactory level.